1. Field of the Invention
The present invention relates to a high-temperature superconductor that has very low superconducting anisotropy, a high superconducting transition temperature (Tc), a high critical current density (Jc), a high irreversible field (Hirr), and a long coherence length .xi. in a direction perpendicular to the plane (the direction being the c-axis direction, the intrafacial plane the ab-axis), and to a method for producing the superconductor.
2. Description of the Prior Art
A high Tc has been considered to be closely related to high superconducting anisotropy (two-dimensionality) of superconducting properties. Known high-temperature superconductors having a two-dimensional layered structure comprised of charge reservoir layers and superconducting layers include Y, Bi, Tl, and Hg based copper oxide superconductors. However, owing to the high superconducting isotropy of these superconductors, at 77 K they do not have a sufficiently high superconductivity, impeding progress toward their practical application at liquid nitrogen temperatures.
In existing superconductors having a layered structure, owing to the large radius of the Ca ions in the Cu.sub.n Ca.sub.n-1 O.sub.2n of the superconducting layers the superconductive coupling between the CuO.sub.2 layers is low. Moreover, because in these superconductors having a layered structure the charge reservoir layer is an insulation layer or non-superconducting layer with low superconductive coupling in the c-axis direction and, therefore, the interactive effect between superconducting layers is small, in addition to which owing to the thinness of the superconducting layers, superconducting anisotropy .gamma. is large, being in the order of 5 to 300 (.gamma. defined as the ratio of the coherence length, the square root of the electron effective mass ratio, or magnetic field penetration depth ratio, is .gamma.=.xi.ab/.xi.c=(mc/mab)1/2+L =.lambda.c/.lambda.ab).
As such, the Jc, especially the Jc under a high magnetic field, and the Hirr, the upper limit of the magnetic field at which zero electrical resistance is generates, become small, posing many problems to the practical use as wire or bulk superconducting material. Furthermore, the large superconducting anisotropy means that the (Jc)c in the c-axis direction is small and the coherence length .xi.c in the c-axis direction also is small, so that when used as a superconducting device material, the properties of the layered structure superconducting device are not adequate, especially the Josephson current density.
An object of the present invention is to provide a high-temperature superconductor that is low in superconducting anisotropy, and a method for producing the superconductor.